Australia: The Land Where Time Began

A biography of the Australian continent 

Alcoota Local Fauna - Late Miocene

This site is less than 200 km south of Alice Springs, 800 km southeast of Bullock Creek and 600 km southwest of Riversleigh, making it the apex of a triangle of Miocene age fossil deposits in northern Australia. They are about half way between the South Australian sites from the Middle Tertiary and the Bullock Creek and Riversleigh sites on the northern limestone plateau. The local fauna is the only known example of a Late Miocene vertebrate community from central Australia. It is the oldest known Australian megafauna of truly large species, that were comparable to the latest Tertiary and Quaternary. At Alcoota, the fossil deposits are in part of the Waite Formation, fine sandstone that ranges in colour from reddish-brown to greyish-green. This formation is made up of sandstones, siltstones and limestones, these layers being above reddish basal laterites. The cycles of erosion, quiescence and sedimentation, sandwiched between the Alcoota bone bed and Ongeva bone bed. These fluviolacustrine sediments were first deposited in the early Tertiary, in a structural basin, that was horseshoe-shaped, between the Harts Range and the hills of the central Australian crystalline basement complex, of Proterozoic age. White silcrete capped the upper part of the formation. This silcrete was used by the Aboriginal People to make stone implements. 25-30 m below the mesa tops on Gidgee plain, that is flat to undulating, the Alcoots Beds crop out. The fossil material from this site are fragile and difficult to extract.

The large bones of the marsupials and the dromornithids are scattered randomly, and some are in entangled masses.

The fossil beds were formed by what appears to be a series of spring-fed lakes that were connected intermittently. There seem to have been dry periods. The concentration of fossils indicate that a condition existed, called waterhole tethering, where water is scarce in the surrounding areas, forcing the fauna of the surrounding area to aggregate around the diminished water bodies. The deposits are believed to have been laid down episodically in channels, a process characteristic of semiarid climates, where the ground cover is sparse and rainfall is seasonal or irregular. At the present, similar sedimentary regimes can be seen in the river courses of the region. Some of these rivers are the Sandover River, the Finke River, and the Todd River at Alice Springs (Murray & Megirian, 1992).

The Alcoota Fossil Beds are on Alcoota Station in central Australia. At about 8 million years old, based on the level of the Diprotodontid fauna present, making it 2-4 million years younger than Bullock Creek Local Fauna, is composed of a series of lenses on a single horizon. Each lens is about 1 m wide and about 170 m long. Birds, crocodiles and marsupials, including a number of Diprotodontidae,  are found in this site. Its age is between that of the older sites in the Bullock Creek in South Australia and Katjamparu in the Northern Territory, and the younger site at Riversleigh in Queensland.

This site, and the Bullock Creek site, show further evidence of the progressing aridification of the continent.

The diprotodontids and macropodids had both underwent a large size increase over their ancestors from the Middle Tertiary. Such as diprotodontoids Palorchestes painei, the marsupial tapir,  Kolopsis torus. Found at this site are the diprotodontoids Alkwertatherium webbi, Plaisiodon centralis and Pyramios alcootense, and the kangaroos Dorcopsoides and Hadronomas. It has been suggested that the trend of these marsupial groups to increase in size in the Late Cainozoic may have resulted from the selection pressure applied to animals by the decreasing nutritive value of their food plants. This can occur because larger animals have slower metabolisms, so they can survive on less nutritious food plants. At any particular site in the late Cainozoic, diprotodontoids are always larger than  the kangaroos at that site. It is thought that this could be a result of the kangaroos being more efficient users of the environment. This may have been because they could breed faster and/or were more efficient at moving about foraging for food in the hard times. They eventually outsurvived the diprotodontoids of similar size.

Thylacinus potens and Wakaleo alcootaensis were the 2 largest marsupial carnivores, both the size of a wolf, and both rare at this site. There was also a fox-sized thylacine in the local fauna. The Baru crocodile was the commonest, and largest, of the fossil carnivores at the site. Claws were found from a very large varanid (monitor lizard), that are identical to those of Megalania prisca (Hecht, 1975).

Also in the Alcoota deposit is the earliest known recognisable Sthenurine, browsing kangaroo, Hadronomas puckridgi (Murray, 1991). The fauna changed little for the first 10 million years of the Miocene, evolving rapidly in the Late Miocene and Early Pliocene.

It has been suggested in a monograph (Michael Woodburne, 1967) that the Alcoota palaeohabitat must have been a forest, possibly a rainforest, based on the large browsing component of the fauna, something completely different from the sparsely wooded area of the present, dominated by Gidgee savanna. On Alcoota Station there is a riparian woodland along Ongeva Creek, and in other parts of the station there are a number of relict dry rainforest shrubs. Among these are Atalaya, Capparis, Ficus, and Grevillea, making up about 10 % of the woody genera in a vegetation dominated by arid-adapted genera such as acacias, eucalypts, as well as forbs (Low, Foster & Smith, 1990). Remnants of an earlier, wetter phase in an otherwise arid zone.

In the Late Pleistocene, the diprotodontoids and kangaroos reached their maximum size. The dromornithids reached their maximum size at an earlier stage of their history, in the Miocene. The first known records of the dromornithids are some tracks in sediments from the Oligocene at Pioneer in Tasmania, and fossils at Bullock Creek and Riversleigh. The largest size and greatest diversity of the dromornithids occurs in the Alcoota deposit from the Late Miocene. The largest was Dromornis stirtoni, the largest known bird, even larger than the elephant birds of Madagascar. In the following epochs they were reduced to a single species in the Pliocene, Dromornis australis, and 1 species in the Pleistocene, Genyornis newtoni.

The Alcoota deposit records a transition in the terrestrial mammalian fauna of Australia. One species that is found in the deposit is of the older fauna, Wakaleo, the marsupial lion. 4 other genera at the site are found only at younger sites elsewhere. Thylacinus, the Tasmanian wolf, the diprotodontoids Kolopsis torus and Palorchestes  painei, and the ringtail possum Pseudochirops. The rest of the fossils at the site are either endemic to the site, or found in only a few sites of similar age. The deposit at Alcoota is the only known site in which 1 element of the old fauna and few of the new fauna groups, that would come to dominate future faunas, are found together.

Sources & Further reading

1. Peter F. Murray & Patricia Vickers-Rich, Magnificent Mihirungs: The Colossal Flightless Birds of the Australian Dreamtime, Indiana University Press, 2004

Links

Thylacinus potens

• Marsupial Fauna
• Fossil Sites